1. Field of the Invention
The invention relates to an integrated process for producing diesel fuel from bitumen and hydrocarbons synthesized from natural gas. More particularly, the invention relates to an integrated process in which a natural gas conversion process produces steam, a high cetane number diesel fraction and hydrogen, wherein the steam is used for bitumen production, the hydrogen is used for bitumen conversion and the diesel fraction is blended with a low cetane number diesel fraction produced from the bitumen.
2. Background of the Invention
Very heavy crude oil deposits, such as the tar sand formations found in places like Canada and Venezuela, contain trillions of barrels of a very heavy, viscous petroleum, commonly referred to as bitumen. The bitumen has an API gravity typically in the range of from 5xc2x0 to 10xc2x0 and a viscosity, at formation temperatures and pressures that may be as high as a million centipoise. The hydrocarbonaceous molecules making up the bitumen are low in hydrogen and have a resin plus asphaltenes content as high as 70%. This makes the bitumen difficult to produce, transport and upgrade. Its viscosity must be reduced in-situ underground for it to be pumped out (produced); it needs to be diluted with a solvent if it is to be transported by pipeline, and its high resin and asphaltene content tends to produce hydrocarbons low in normal paraffins. Underground bitumen is generally produced by steam stimulation, in which hot steam is injected down into the formation to lower the viscosity of the oil sufficient to pump it out of the ground. This is disclosed, for example, in U.S. Pat. No. 4,607,699. In U.S. Pat. No. 4,874,043 a method is disclosed in which both hot steam and hot water are alternately pumped into the ground. A significant requirement of steam stimulated bitumen production is a source of readily available steam, most of which is lost or consumed in the process and cannot be recovered. As a consequence of the relatively low hydrogen content of the bituminous molecules, diesel fuel produced by coking and hydrotreating bitumen tends to be low in cetane number. Hence, when bitumen diesel production is desired, a higher cetane hydrocarbon blending component is needed to mix with the lower cetane bitumen diesel.
Gas conversion processes, which produce hydrocarbons from a synthesis gas derived from natural gas, are well known. The synthesis gas comprises a mixture of H2 and CO, which are reacted in the presence of a Fischer-Tropsch catalyst to form hydrocarbons. Fixed bed, fluid bed and slurry hydrocarbon synthesis processes have been used, all of which are well documented in various technical articles and in patents. Both light and heavy hydrocarbons may synthesized, including diesel fractions relatively high in cetane number. In addition to hydrocarbon production, these processes also produce steam and water. It would be an improvement to the art if bitumen production and gas conversion could be integrated, to utilize features of the gas conversion process to enhance bitumen production and products and produce a diesel fuel fraction having a cetane number higher than is obtained just from bitumen.
The invention relates to a process in which natural gas is converted to a synthesis gas feed, from which liquid hydrocarbons, including a diesel fraction are synthesized and steam is generated, to facilitate bitumen production improve the cetane number of diesel produced from the bitumen. The conversion of natural gas to synthesis gas and the synthesis or production of hydrocarbons from the synthesis gas will hereinafter be referred to as xe2x80x9cgas conversionxe2x80x9d. The natural gas used to produce the synthesis gas will typically and preferably come from the bitumen field or a nearby gas well. The gas conversion process produces liquid hydrocarbons, including a diesel fraction, steam and water. The steam is used to stimulate the bitumen production and the higher cetane number gas conversion diesel is blended with the lower cetane number bitumen diesel, to produce a diesel fuel stock. Thus, the invention broadly relates to an integrated gas conversion and bitumen production and upgrading process, in which gas conversion steam and diesel fraction hydrocarbon liquids are respectively used to stimulate bitumen production and upgrade a bitumen-derived diesel fraction. The conversion of natural gas to a synthesis gas is achieved by any suitable synthesis gas process.
The hydrocarbons are synthesized from synthesis gas that comprises a mixture of H2 and CO. This gas is contacted with a suitable hydrocarbon synthesis catalyst, at reaction conditions effective for the H2 and CO in the gas to react and produce hydrocarbons, at least a portion of which are liquid and include a diesel fraction. It is preferred that the synthesized hydrocarbons comprise mostly paraffinic hydrocarbons, to produce a diesel fraction high in cetane number. This may be achieved by using a hydrocarbon synthesis catalyst comprising a cobalt and/or ruthenium, and preferably a cobalt catalytic component. At least a portion of the gas conversion synthesized diesel fraction is upgraded by hydroisomerization to lower its pour and freeze points. The higher boiling diesel hydrocarbons (e.g., 500-700xc2x0 F.) are highest in cetane number and are preferably hydroisomerized under mild conditions, to preserve the cetane number. The gas conversion portion of the process produces high and medium pressure steam, all or a portion of which are injected into the ground to stimulate the bitumen production. Water is also produced by the hydrocarbon synthesis reaction, all or a portion of which may be heated to produce steam for the bitumen production, for utilities or both. Thus, by xe2x80x9cgas conversion steamxe2x80x9d or xe2x80x9csteam obtained or derived from a gas conversion processxe2x80x9d in the context of the invention is meant to include any or all of the (i) high and medium pressure steam produced by the gas conversion process and (ii) steam produced from heating the hydrocarbon synthesis reaction water, and any combination thereof. Still further, a methane rich tail gas is also produced by the gas conversion process and may be used as fuel, including fuel for utilities and to produce steam from the synthesis reaction water and/or further heat the gas conversion steam. By bitumen production is meant steam stimulated bitumen production, in which steam is injected down into a bitumen formation, to soften the bitumen and reduce its viscosity, so that it can be pumped out of the ground.
Upgrading comprises fractionation and one or more conversion operations. By conversion is meant at least one operation in which at least a portion of the molecules is changed and which may or may not include hydrogen as a reactant. If hydrogen is present as a reactant it is broadly referred to as hydroconversion. For the bitumen, conversion includes cracking, which may be coking (non-catalytic) or catalytic cracking, as well as hydroconversion, as is known and explained in more detail below. In another embodiment of the invention, hydrogen useful for converting the synthesized hydrocarbons is produced from the synthesis gas generated in the gas conversion portion of the process. The hydrocarbon synthesis also produces a tail gas that contains methane and unreacted hydrogen. In a further embodiment, this tail gas may be used as fuel to produce steam for bitumen production, pumps or other process utilities.
The process of the invention briefly comprises (i) stimulating the production of bitumen with steam obtained from a natural gas fed gas conversion process that produces a diesel hydrocarbon fraction and steam, (ii) converting the bitumen to form lower boiling hydrocarbons, including a diesel fraction, and (iii) forming a mixture of the gas conversion and bitumen diesel fractions. In a more detailed embodiment the invention comprises the steps of (i) producing bitumen with steam stimulation, (ii) upgrading the bitumen to lower boiling hydrocarbons, including a sulfur-containing bitumen diesel fraction, (iii) treating the bitumen diesel fraction to reduce its sulfur content, (iv) producing steam and hydrocarbons, including a diesel fraction, by means of a natural gas fed gas conversion process, wherein at least a portion of the steam is used for the bitumen production, and (v) treating at least a portion of the gas conversion diesel fraction to reduce its pour point. At least a portion of both treated diesel fractions are then blended to form a diesel stock. In a still more detailed embodiment the process of the invention comprises:
(i) converting natural gas to a hot synthesis gas comprising a mixture of H2 and CO which is cooled by indirect heat exchange with water to produce steam;
(ii) contacting the synthesis gas with a hydrocarbon synthesis catalyst in one or more hydrocarbon synthesis reactors, at reaction conditions effective for the H2 and CO in the gas to react and produce heat, liquid hydrocarbons including a diesel fuel fraction, and a gas comprising methane and water vapor;
(iii) removing heat from the one or more reactors by indirect heat exchange with water to produce steam;
(iv) hydroisomerizing at least a portion of the diesel fraction to reduce its pour point;
(v) passing at least a portion of the steam produced in either or both steps (i) and (iii) into a tar sand formation to heat soak and reduce the viscosity of the bitumen;
(vi) producing the bitumen by removing it from the formation;
(vii) upgrading the bitumen to lower boiling hydrocarbons, including a diesel fuel fraction containing heteroatom compounds;
(viii) hydrotreating the bitumen diesel fuel fraction to reduce its heteroatom content, and
(ix) combining at least a portion of each of the treated diesel fuel fractions.
The hydrotreating also reduces the amount of unsaturated aromatic and metal compounds. By bitumen diesel fraction referred to above is meant a diesel fuel fraction produced by upgrading the bitumen including coking and fractionation. The tar sand formation is preferably an underground or subterranean formation having a drainage area penetrated with at least one well, with the softened and viscosity-reduced bitumen produced by removing it from the formation up through the well.